1. Field of the Invention
This invention relates to glass fibers coated with an amorphous metallic alloy, and, more particularly, to such a coated fiber useful as an optical waveguide.
2. Description of Related Art
Optical waveguides transmit light or other radiation from one point to another through the waveguide, with little loss of light intensity during transmission. One type of optical waveguide is manufactured as a glass fiber which has a central core of a doped glass having a high index of refraction and an annular outer cladding of glass having a lower index of refraction. Conversely, the doped glass may comprise the outer cladding with the central core undoped. In either case, light travels primarily along the optical waveguide in the central core. Light beams which tend to stray from the central core because of a bend in the optical waveguide or for other reasons are reflected back into the central core. The internal refraction is highly efficient, so that the over all transmission of light via the optical waveguide occurs with very little loss.
Optical waveguides are typically fabricated by first preparing a preform having a relatively large overall diameter, such as about 1 centimeter. The preform includes a core of one type of glass and an outer cladding of the other type of glass. The preform is usually prepared by doping the inside surface of a hollow glass tube, or by placing a glass tube over a glass rod. This preform is then heated and drawn to a very fine fiber diameter, such as about 0.1 millimeter. Such very fine glass fibers are strong, but are easily damaged by abrasion or scratching of the surface of the fiber and can be highly susceptible to stress corrosion damage. It is therefore important to protect the surface of the fiber from surface damage, and various types of protective materials have been utilized for this purpose. It has been proposed to coat the outer surface of glass fibers with a metallic coating, which would protect the surface of the fibers and also provide a significant hermetic jacket. See, for example, U.S. Pat. Nos. 4,418,984 and 4,407,561.
Optical waveguides are primarily utilized in two types of applications. In the first, the purpose of the waveguide is simply to transmit light from one point to another, with little loss of intensity, for the purpose of communications. In the second, the waveguide is used as a sensor element to detect the presence and strength of some external force or field by its effect on the light as it travels through the optical waveguide. In one such application, an optical waveguide fiber may be coated with a magnetostrictive metal so that the presence of small external magnetic fields is detected in phase shifting of the light travelling through the optical waveguide. The external magnetic field causes a magnetostrictive contraction in the external coating of metal, and this contraction alters the phase relationships of the light traveling through the optical waveguide, so that a correlation between the external magnetic field and the phase relationships may be made.
One class of candidates for metallic coatings for use on optical waveguides is amorphous, or glassy, metallic alloys. Amorphous metals have no long-range order, and therefore do not exhibit any crystalline structure, unlike most solid metals. Amorphous materials have been observed to be strong, of acceptable ductility for their strength, fatigue resistant, and corrosion resistant. Additionally, certain amorphous metallic alloys are magnetostrictive, or have other properties which are attractive in specific optical waveguide applications. Thus, it would be highly desirable to utilize amorphous metals as coatings for certain optical waveguide fibers.
The fabrication of amorphous metallic alloys requires specialized procedures wherein the metallic atoms are prevented from forming on a crystalline lattice in the usual fashion. In one fabrication approach, a liquid metal alloy is solidified at a very high cooling rate, on the order of 10.sup.5 .degree.C. per second. It has also been demonstrated that amorphous metallic alloys may be prepared by electrodeposition, vapor deposition, and similar specialized techniques.
Optical waveguide fibers having an amorphous metallic coating have been prepared by vapor depositing amorphous metal on the glassy surface, for a demonstration of magnetostrictive effects. However, this approach cannot be readily utilized for manufacturing long lengths of coated optical waveguides, since the vapor deposition process inherently is relatively slow. Coating by vapor deposition produces a very thin coating subject to failure. Existing vapor deposition techniques use apparatus which does not coat the fibers immediately after the glass surface is formed, so that the fiber surface may be damaged or degraded before it can be coated. Thus, vapor deposition techniques are not suitable for preserving the nascent strength of glass fibers coated with amorphous metals.
In an alternative approach, it has been proposed that coated optical waveguide fibers could be prepared by placing a layer of a non-amorphous metal alloy onto the surface of a preform prior to drawing the rod to a fiber. The coated preform would then be heated to a temperature greater than the melting point of the metallic alloy and rapidly drawn to a fiber by a die-less drawing procedure. It was proposed that, during the die-less drawing procedure, both the glass fiber and the outer metallic layer would be cooled so rapidly that the outer metallic layer would transform to its amorphous state, resulting in a glass fiber having external coating of an amorphous metallic alloy.
This proposed approach suffers from major drawbacks which make it impractical for preparing long lengths of coated optical waveguide fibers. First, the molten metal may simply melt and run off the fiber during processing. Second, the molten metal and the glass would be in contact for a relatively long period of time, between melting and solidification of the metal, resulting in extensive chemical attack of the glass. Third, it is believed that the procedure simply is inoperable in most situations to produce an amorphous outer coating, as the necessary high cooling rate cannot be achieved by convection and radiation of heat to the ambient atmosphere. Fourth, in this proposed approach, the drawing and elongation of the central glass fiber should continue after the metallic coating has solidified, but in fact the solidified metal prevents further drawing, or the ductility of the metallic coating is insufficient to maintain coating integrity during the drawing process. The amorphous metallic coating is therefore quite likely to fracture or interrupt the fiber drawing process. Fifth, the drawing temperatures of most glasses of interest in waveguide applications are not campatible with the proposed approach, and the drawing temperatures are usually much higher than the solidification temperatures of the glass-forming metallic alloys. Finally, it is particulary desirable that the metallic coating be placed on the glass fibers immediately after the fiber surface is formed, a condition not achieved by this described process.
No other techniques for forming an amorphous metal coating on a glass fiber are known. Accordingly, there exists a need for a process and apparatus for coating glass fibers with an amorphous metallic coating, immediately after the fiber surface is formed. Such a process and apparatus should provide for uniform coating of the optical waveguide fiber about its entire circumference, should have conditions favorable to producing an amorphous coating structure, and should allow the rapid, continuous and economic manufacture of long lengths of coated optical waveguide fiber by application of the amorphous metal to the pristine surface of the fiber immediately after the fiber is formed. The present invention fulfills this need, and further provides related advantages.